This invention relates generally to power plants. More specifically, the invention relates to an improved heat source for use in indirect gas turbine power plants.
In so-called direct gas turbine power plants, the products of combustion, including any unburned fuel, actually pass through the turbine. Because of this, only very clean fuels, such as natural gas and other distillate fuels, can be used in order to prevent damage to the turbine blading. The products of combustion of less refined fuels, such as biomass, coal or wood wastes, cannot be directly used to fuel a gas turbine since the gaseous products of combustion of these fuels often contain particulates which may destroy or cause undue wear of the expansion section of a gas turbine. Moreover, these less refined fuels often provide an insufficient degree and rate of expansion within the gas turbine, thereby seriously limiting the energy output of the turbine.
Given the dwindling supply of fossil fuels, it is imperative that maximum use of all types of fuels be achieved. In order to make use of lower-grade fuels in so-called indirect gas turbine power plant systems, a turbine working fluid, such as air, is utilized to absorb heat from the fuel via various types of heat exchangers. The working fluid and the heat contained therein is subsequently passed through the turbine in order to generate electrical and/or mechanical power. Since the products of combustion do not actually pass through the gas turbine, these indirect systems permit the use of fuels such as biomass, coal and wood wastes. As used herein, the term "biomass" is intended to define any type of combustible plant material, including organic fibrous materials such as sewer sludge. The term "wood wastes" defines wood materials such as bark, shavings, trimmings, chips, sawdust, hog fuel, and the like which are typically by-products of various operations performed at a lumber mill. Biomass, coal and wood wastes are typically in particulate form when they are to be used as fuel in a suspension burner.
One serious disadvantage of conventional indirect systems is that a considerable amount of energy is lost in attempting to transfer heat from the burning fuel to the turbine working fluid. Many suggestions have been made to increase heat transfer efficiency in such indirect systems. For example, heat exchangers have been placed inside high temperature combustion chambers, where they are subjected to the heat of ongoing combustion. One such system is disclosed in U.S. Pat. No. 2,434,950 to Nettel. Nettel also includes a second heat exchanger positioned remote from the combustion chamber in an attempt to absorb additional heat from the products of combustion leaving the combustion chamber. The term "products of combustion" as used herein is intended to cover not only the exhaust gases which result from combustion of fuel, but also gaseous combustibles in the form of vapors which may emanate from liquid or solid fuels, and air, which, for various reasons, has not been used up in combustion. Nettel also suggests that the products of combustion might be combined with hot air exhausted from the turbine to form a stream which is then fed back to the combustion chamber as combustion air. While Nettel's system is undoubtedly more efficient than some other designs, he overlooks the fact that the fuel is not going to be completely burned in his single combustion chamber. For example, as noted above, combustible (and therefore energy-containing) gases will typically be emitted with the exhaust gases from this combustion chamber. Therefore, Nettel's system inherently wastes fuel which will eventually pass to the atmosphere with the other products of combustion. A second drawback with Nettel's system is that he positions a heat exchanger within his primary combustion chamber. This requires the use of exotic and therefore expensive materials since the heat exchanger will be subjected to extremely high temperatures.
The present invention overcomes these disadvantages in Nettel's and other prior art systems by providing a unique arrangement of a secondary combustion chamber having a heat exchanger therein. The use of secondary combustion chambers per se is not new. For example, my earlier U.S. Pat. No. 3,831,535 discloses a so-called blending chamber which receives products of combustion from a primary combustion chamber, and continues to burn those products of combustion along with recirculated volatile fumes which provide additional fuel to the blending chamber. However, neither the system disclosed in my aforementioned patent, nor any other prior systems, include the unique features found in the present invention which permit greater efficiency than previously thought possible.
It is therefore an object of the present invention to increase the efficiency of combustion systems utilizing biomass, coal, wood wastes, and other types of fuels, for use with power plants, compressors, or other engines. Another object is to provide a power plant system wherein temperatures throughout the system are easily controllable in order to maximize the efficiency of the system and to maintain the desired output temperatures and pressures. Yet another object is the provision of a system in which the primary combustion chamber is maintained at an optimum temperature for a more complete combustion of particulate fuels. Another object is to utilize energy derived from the combustion of wood wastes and other fuels to generate electricity using a conventional gas turbine, air to air heat exchanger, and electrical generator. A more specific object is to provide a power plant system especially adapted for optimum utilization of the potential heat energy from the combustible wood wastes of a lumber or plywood mill to generate electricity for powering both in-plant and out-of-plant apparatus.
This invention responds to the problems presented in the prior art by providing an indirect gas turbine power plant having primary and secondary combustors wherein fuel is burned and heat is conveyed to a turbine working medium which is subsequently passed through the turbine section of a gas turbine. The gas turbine includes both a compressor section and a turbine section. The primary combustor has a first inlet for receiving exhaust air from the turbine section, a second inlet for receiving fuel and an outlet for discharge of products of combustion. The secondary combustor includes a first inlet for receiving at least a portion of the products of combustion from the primary combustor, a second inlet for receiving a portion of the products of combustion of the secondary combustor, and an outlet for discharge of the products of combustion of the secondary combustor. A gas-to-gas heat exchanger, hereinafter referred to as an air heat exchanger, is positioned within the secondary combustor. This heat exchanger includes an inlet for receiving compressed air from the compressor section of the gas turbine, heat exchange surface means for conveying heat from the products of combustion to the compressed air, and an outlet to direct the compressed, hot air out of the secondary combustor for passage to the turbine section of the gas turbine. First conduit means are also included for conveying a portion of the products of combustion from the secondary combustor outlet to the second inlet of the secondary combustor. This first conduit may, if desired, direct the secondary combustor products of combustion through a boiler or other heat use before conveying the products of combustion back to the secondary combustor. Also, the first conduit may be adapted to receive other energy inputs such as hot exhaust air or other gases which would be combined with the secondary combustor products of combustion prior to reinjection back into the secondary combustor.
The invention also normally includes a boiler having inlet means for receiving hot gases which include exhaust air from the turbine section of the gas turbine and the products of combustion of the secondary combustor, outlet means for discharging the hot gases, and steam generation means for conveying heat from the hot gases to water within the boiler and thereby generating steam.
It may be desirable in certain systems to eliminate the gas turbine from the power plant. In such systems the compressed, hot air which is discharged from the secondary combustor air heat exchanger can be directed to energy absorption means which would use a portion of the energy contained in such air and pass the remainder to the primary combustor for use as combustion air, to a boiler, or to both the primary combustor and a boiler. Such energy absorption means might typically comprise a dehydrator for food processing and the like, a glass production facility, or any other use which requires substantial amounts of heat energy. Of course, when the gas turbine is eliminated from the system, the air compressor would be driven by other means.